FINDINGS Full Scale IQ

Tables 4.1 and 4.2 display the summary “vote count” for relationships between measures of intelligence and violent versus nonviolent antisocial behavior. For full scale IQ scores, a very strong preponderance of comparisons (PoC3) in studies of violent behavior is in the predicted direction (an inverse relationship) (k = 15 studies out of 19) with most PoC being statistically significant (k = 10). There is a paucity of studies reporting associations between IQ and strictly nonviolent offending (k = 4) (recall that we exclude all studies of general offending which combined violent and nonviolent offending). A preponderance exists in the predicted direction but there are too few studies to draw firm conclusions.

There are many studies that compared intellectual abilities of violent and nonviolent offenders, and a strong majority found that violent offenders had lower scores than nonviolent offenders.

Verbal Ability and Violent Behavior

There were 24 studies out of 29 that reported a preponderance of comparisons in the predicted direction (see Table 4.1). In 11 out of 29, the preponderance was also statistically significant (see also Table 4.2 for comparison-l evel findings). This is strong evidence that verbal ability is correlated with violence and confirms our earlier conclusions and those of many other reviewers. In addition, 21 out of 26 studies comparing violent to nonviolent offenders found that violent offenders had lower scores on verbal ability, and 9 of those studies reported a PoC that was also statistically significant. The proportion of statistically studies reporting significant differences is much greater than we would expect by chance (which would be less than 5% if a = .05). There were too few coefficients representing associations with strictly nonviolent offending to draw any conclusions; the PoCs were divided between 3 in the predicted direction and 2 in the wrong direction.

Table 4.1 Summary of Study-Level Findings Related to Measures

of Intelligence and Violent and Nonviolent Offending

Summary of STUDY Results

Independent Variable Category

Number of Studies (k)

В

О

©

X

Full Scale IQ

Violent Nonviolent Violent vs. Nonviolent

  • 19
  • 4
  • 26
  • 3
  • 0
  • 3
  • 0
  • 1
  • 5
  • 1
  • 0
  • 0
  • 5
  • 2
  • 5
  • 10
  • 1
  • 13

Verbal Ability

Violent Nonviolent Violent vs. Nonviolent

  • 29
  • 5
  • 26
  • 3
  • 0
  • 0
  • 1
  • 2
  • 5
  • 1
  • 0
  • 0
  • 13
  • 1
  • 12
  • 11
  • 2
  • 9

Performance

Intelligence (PIQ)

Violent Nonviolent Violent vs. Nonviolent

  • 9
  • 1
  • 17
  • 1
  • 0
  • 1
  • 0
  • 1
  • 2
  • 1
  • 0
  • 0
  • 4
  • 0
  • 6
  • 3
  • 0
  • 8

Deviation Score (PIQ-VIQ)

Violent Nonviolent Violent vs. Nonviolent

  • 2
  • 0
  • 5
  • 0
  • 0
  • 0
  • 0
  • 0
  • 1
  • 1
  • 0
  • 1
  • 0
  • 0
  • 2
  • 1
  • 0
  • 1

Щ Findings are ambiguous

О Findings are in the opposite direction of the attachment hypothesis (not necessarily statistically significant)

© Relationship is reported as “null” or coefficient = 0

• Findings are in the expected direction of the attachment hypothesis but are not statistically significant

X Findings are in the expected direction of the attachment hypothesis and are statistically significant

 
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